Engine parameters such as air-fuel ratio (AFR) can be controlled to ensure improved engine performance leading to effective use of an exhaust catalyst and reduced exhaust emissions. In particular, cylinder-to-cylinder imbalances in air-fuel ratio can lead to inefficient engine operation and an increase in engine-out emissions. In addition, there may be torque imbalances between the engine cylinders which can result in NVH issues.
One way to determine AFR variation between engine cylinders is to sense engine exhaust gases via an oxygen sensor located downstream of an exhaust catalyst. By measuring the exhaust gas components, it may be determined if a given cylinder is running richer or leaner than other cylinders. Fuel and/or charge air parameters may then be adjusted based on the variation to produce an air-fuel mixture at a target air-fuel ratio. However, the oxygen sensor may be exposed to exhaust gases that are a combination of gases from different engine cylinders. Therefore, it may be difficult to accurately determine air-fuel variations between different engine cylinders. Further, engine exhaust system geometry for cylinders having a large number of cylinders may bias sensor readings toward output of one cylinder more than other cylinders. Consequently, it may be even more difficult to determine air-fuel imbalance for engines having more than a few cylinders. Still other approaches may include monitoring torque pulses on the crankshaft (or monitoring crankshaft acceleration at a desired AFR) and deriving a correlation between torque amplitude and combustion air-fuel ratio. However, in all of these approaches, it may be difficult to differentiate the air component of the error from the fuel component of the error.
One example approach for learning air-based errors is shown by Gottschalk et al in U.S. Pat. No. 9,470,159. Therein, a direct fuel injector is actuated open to deliver fuel into a cylinder. A drop in direct injection fuel line pressure is measured while the injector is open and is used, in addition with a transfer function, to estimate the air charge amount in the cylinder. By comparing the air charge estimated in this way for each cylinder, the air component of cylinder-to-cylinder AFR or torque variations can be learned.
However, the inventors herein have recognized potential issues with such an approach also. As one example, the estimation may be limited by the fuel line pressure sensor's range of resolution. For example, at low engine loads, when the fuel line pressure is low, the drop in fuel line pressure may not be significant enough to be reliably measured by the sensor. As another example, the measured drop in fuel line pressure may be affected by the location of the piston in the cylinder, specifically, based on whether the piston is at TDC or BDC of a compression stroke. As yet another example, it may be difficult to differentiate the drop in fuel line pressure due to a fuel-based error from the drop due to an air-based error.
In addition, exhaust gas recirculation (EGR) flow can corrupt the fuel pressure sensor output, and the air flow estimated based on the fuel pressure sensor output. In particular, based on the configuration of the intake manifold, as well as the intake location where the EGR is received, different cylinders may get different EGR flows, affecting individual cylinder air charge estimations.
The inventors herein have recognized the shortcomings discussed above and have developed a method for determining air-fuel ratio imbalance and air-based error in engine cylinders taking into account AFR variations among cylinder groups. In one example, AFR imbalance may be determined by a method for an engine, comprising: injecting fuel from a direct injector, with a high pressure pump disabled, to reduce a direct injection fuel rail pressure below a threshold pressure; and then, injecting fuel into a cylinder and commanding the direct injector to selectively open a threshold duration before a spark event in the cylinder, without injecting any fuel from the direct injector. In this way, an air component of a cylinder AFR variation may be accurately learned and reliably differentiated from a fuel component of the AFR variation.
As one example, when operating a port fuel direct injection (PFDI) engine in a PFI only mode, an engine controller may estimate a compression pressure of the cylinder via a pressure sensor coupled to a high pressure direct injection (DI) fuel rail. The estimated compression pressure may then be used to infer the air charge of the cylinder. Specifically, the controller may disable a high pressure pump (HPP) coupled to the DI fuel rail and then, before injecting fuel via the port injector, inject fuel via the direct injector to bleed the high pressure fuel rail to a threshold pressure (e.g., to a lower threshold). Then, port fuel injection may be enabled and immediately before spark is delivered to the cylinder, the DI may be commanded open for a defined (short) duration. The high pressure fuel rail may become coupled to the cylinder, transiently, when the direct injector is opened, allowing the compression pressure in the cylinder to be estimated via the pressure sensor coupled to the high pressure fuel rail. In particular, the compression pressure may be noted as a transient spike in the fuel rail pressure. Since the compression pressure is directly related to the cylinder volume and the amount of air drawn into each cylinder, the spike in fuel rail pressure may be correlated with the air charge in that cylinder. By continuing this operation until the air charge in each cylinder is estimated, and by repeating this operation several times for each cylinder, a stable average pressure may be obtained for each cylinder. By comparing the values for each cylinder, the air component of cylinder-to-cylinder AFR variations may be learned. By performing the estimation when EGR flow is enabled and when EGR flow is disabled, the noise effect of EGR on the air-based error estimation can be quantified and compensated for. Subsequently, the fuel rail pressure may be used for estimating the fuel component of the AFR variations. Therein, the HPP may be actuated to raise the DI fuel rail pressure to a threshold (e.g., an upper threshold), after which direct injection of fuel into the cylinder may be enabled, and a drop in fuel rail pressure following each injection pulse may be correlated with the pulse-width commanded on each pulse.
In this way, the method provides improved capability for learning air-fuel ratio imbalance. The technical effect of measuring a cylinder compression pressure to estimate cylinder air charge is that an air-based error among cylinder groups may be more accurately learned, and more accurately differentiated from a fuel-based error. By measuring a rise in DI fuel rail pressure during conditions when the cylinder is only fueled with port injection, the effect of the cylinder's compression pressure on the fuel rail pressure can be learned in a stable region of the fuel rail pressure sensor over a wider range of engine loads, including at low engine load. Consequently, the approach ensures improved fuel efficiency and reduced emissions. In addition, the method can compensate for air-fuel ratio imbalance associated with EGR flow, enabling the learning to be performed over a wider range of engine operating conditions, and without compromising EGR usage. By learning the air-based error among cylinder groups, AFR errors may better learned and compensated for.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.